1. Field of the Invention
This invention relates to a self-vibrating dc-operated flat coreless vibrator motor used for a pocketable wireless silent alerting device. The motor does not have an output shaft or an eccentric external weight, but its flat armature rotor has an eccentric mass distribution for creating vibration.
2. Description of the Prior Art
A Japanese laid-open patent application, Laid-open No. 63-290140, dated Nov. 28, 1988, discloses a flat coreless vibrator motor. In the rotor of this vibrator motor, one of conventional three armature coils, which are normally arranged at a pitch angle of 120.degree., is eliminated, and only two coils are partially embedded in a wheel-shaped rotor body made of a synthetic resin. The same Japanese patent publication also discloses an embodiment in which a sector-shaped eccentric weight is mounted on an output shaft of a conventional type rotor having three armature coils arranged at a pitch angle of 120.degree..
U.S. Pat. No. 4,864,276, issued Sep. 5, 1989, discloses a flat coreless vibrator motor that employs a rotor having three armature coils arranged seemingly at an even pitch angle of 120.degree. but one of the three coils is substantially smaller than the other two so that rotor unbalance is effected.
A vibrator motor having an eccentric weight mounted on its rotor shaft requires axially extra dimension and, therefore, the motor can not be designed in a flat profile. The vibrator motor having a circular rotor, in which one of the armature coils is eliminated or minimized, has problems of providing insufficient torque and insufficient weight unbalance, which causes insufficient vibrations.
The present inventor disclosed in U.S. Pat. No. 5,036,239, issued Jul. 30, 1991, a flat coreless vibrator motor that employs a generally sector-shaped rotor having three armature coils arranged in a non-overlapping manner at an equal pitch angle between 70.degree. and 80.degree.. The coils are molded in a rotor body made of a synthetic resin.
FIG. 8 is a plan view of a rotor employed in a prior-known flat coreless vibrator motor. In FIG. 8, a flat generally sector-shaped coreless rotor R4 has a shaft 41, a brass-made toroidal center holder 42, fixedly mounted on the shaft 41, a body 43 made of a synthetic resin fixedly attached to the holder 42, and armature coils 44a, 44b, and 44c molded and partially embedded in the body 43. The armature coils 44a, 44b, and 44c are disposed in a non-overlapping manner at an even pitch angle of 60.degree. about the shaft 41 within a 180.degree. angular area.
A flat sector-shaped rotor provides a good weight unbalance and the vibrator motor disclosed in U.S. Pat. No. 5,036,239 has favorably been accepted in the market. However, the market always demands a smaller portable alerting device, and this trend requires even more compact vibrator motor having sufficient torque and weight unbalance of the rotor.